[css-values-5] Replace the notion of arb-sub functions creating a dependency graph with them tracking a stack of dependencies as they evaluate. (Should be editorial, but it's a deep rewrite.) Give arb-sub functions a 2-stage grammar, allowing them to control when their bits are evaluated. Add spread syntax to arb-sub functions. w3c#11144 w3c#11500 w3c#10679

Author: tabatkins